The developmental transition of the moss gametophyte from filamentous protonema to leafy gametophore is being developed as a model for eukaryotic systems in which large blocks of genes are switched on or off. The transition (in Nanomitrium austinii) may be induced by cytokinins or, under certain conditions and reversibly, by changes in temperature. The coordinate control of the two structural genes of ribulose bis-phosphate carboxylase is also being investigated in these organisms.